1. Field of the Invention
This invention relates in general to apparatus for modifying the time base of signals in a train thereof; more particularly, the invention is concerned with apparatus for correcting a time base instability in television-type signals.
2. Description Relative to the Prior Art
While the invention is not so restricted, its utility as a flutter compensator in a video tape recorder dramatically indicates its significance. The playback of a taped video recording requires faithful replay of recorded signals without causing such signals to shift unwantedly and continuously in frequency. Unwanted frequency modulation of playback video signals can, among other things, cause (1) the sync information content of the video signal to become unstable, and (2) the color information content of the video to become, at best, faulty, i.e., color information may not be detectable at all, since unstable playback of recorded color burst information within the video signal may cause the color content of the video signal to be lost entirely.
The degree of unwanted frequency shift during playback of the video signal is measured in terms of "percent flutter", it being recognized that even fractional percent flutter will prevent the AFC circuits of most televisions from tracking the fluttering sync signal, resulting in the tearing apart of the visual display that corresponds to the playback signal. Percent flutter may be defined as 100 times a change in frequency divided by a nominal frequency, the nominal frequency in the present case being 15750 H2.
To assure against instantaneous time base instability of played-back television signals which are derived from a video tape recorder, present practice is to employ extremely close tolerances in the precision parts of the recorder and to complement such tolerances with various servo controls which actively prevent flutter from occurring during playback. Such practices greatly add to the cost of manufacturing video tape recorders and, indeed, have prevented video recorders from being cost-wise within reach of many who would otherwise want them. This being the case, various proposals have been made to utilize variable delay lines, e.g., U.S. Pat. Nos. 2,960,568; 3,238,300; and 3,580,991, as a way to nullify flutter within playback signals. In one of the later versions, as shown in U.S. Pat. No. 3,580,991, the flutter within the played-back signals is detected and the "flutter" signal derived therefrom is used to vary the delay of a storage-type delay device through which the fluttering signal is passed. Thus, the output of the delay device is, hopefully, a flutter-free equivalent of the fluttering input to the delay device.
The prior art objective of providing a flutter correcting delay device depends on forcing the delay of the fluttering signal through the delay device to vary linearly with the unwanted frequency modulation of the signal. However, considering, for example, a frequency controlled storage-type delay device, the transit time of such a delay device varies nonlinearly with the frequency of the clock synchronizing pulses which clock a signal therethrough. Therefore, to produce the desired result, the delay device must be clocked in accordance with a varying pulse rate which compensates for the inherent nonlinearity of the delay device, thereby causing the delay to assume the desired linear relationship with respect to the unwanted frequency modulation. What this means is this: One cannot merely derive a signal corresponding to the frequency modulation; invert such signal, and then operate the frequency controlled delay device at a rate represented by the inverted signal. To do so, in effect, would merely cause the delay device to track, but never nullify, the unwanted frequency modulation.
Approaches taken heretofore to compensate for the "nonlinearity problem" include such proposals as illustrated in U.S. Patent No. 3,580,991, which requires the superimposition of the outputs from two phase comparator circuits in order to approach the desired result. Besides offering a result which only approximates the desired flutter-free signal, this approach also complicates the circuitry in the recorder by resort to dual comparator units and, of course, raises the cost thereof. Proposals have also been advanced to use one comparator circuit and to convert its analog output to a digital output in order to control a delay network. However, this approach requires expensive and complex analog-to-digital and digital-to-analog converters. Consequently, a quality flutter-free video recorder is still beyond the financial reach of many who would otherwise want one, particularly in a home or educational setting.